Crystal defects are invariably present in a semiconductor layer. The semiconductor layer of III-V semiconductor manufactured by existing manufacturing techniques, for example, has many crystal defects. Most of the crystal defects extend in the direction of thickness of the semiconductor layer.
When semiconductor devices are made from III-V semiconductors such as GaN (gallium nitride) or the like, there is a greater dielectric breakdown field and a higher degree of movement of saturated electrons than in a case where silicon is used. As a result, when semiconductor devices are manufactured from III-V semiconductor layers, semiconductor devices should be realized that have high withstand voltage and are capable of controlling large currents. Furthermore, research is being actively performed on semiconductor devices that have a vertical electrodes structure. Semiconductor devices with the vertical electrodes structure have the advantage that insulation can easily be maintained between the pair of main electrodes. Further, with semiconductor devices that have the vertical electrodes structure, a wiring layout connected to the pair of main electrodes can be simplified. As a result, semiconductor devices with the vertical electrodes structure have the advantage that it is easy to reduce a distance of wiring formed on a base plate on which the semiconductor device is mounted. When the distance of the wiring is reduced, resistance of the wiring can be reduced.
As described above, existing III-V semiconductor layers contain many crystal defects. Under current conditions, where crystal defects are present, the issue as to how semiconductor devices with excellent characteristics can be manufactured is consequently an important one. Reducing the effects on characteristics of semiconductor device caused by crystal defects is extremely important when III-V semiconductor layers are used. However, this object is not limited to the case where III-V semiconductor layers are used. Reducing the effects on characteristics of semiconductor device caused by crystal defects is widely needed in various semiconductor materials.
In the technique taught in Japanese Laid-Open Patent Application Publication No. 2001-230410, a semiconductor device is manufactured by using so-called epitaxial lateral overgrowth method. In the technique taught in Japanese Laid-Open Patent Application Publication No. 2001-230410, the epitaxial lateral overgrowth method is executed after a mask that has openings has been formed on a base plate. When the epitaxial lateral overgrowth method is adopted, crystals grow from the base plate exposed at the openings of the mask in a direction perpendicular to a surface of the base plate. In a region covered by the mask, the crystals grow in a direction parallel to the surface of the base plate. This method allows crystal defects to be reduced in the region where the crystals have grown in the direction parallel to the surface of the base plate. However, many crystal defects are formed in the region where the crystals have grown in the direction perpendicular to the surface of the base plate. Moreover, it is not possible to grow crystals in the direction parallel to the surface of the base plate without also having the region where the crystals grow in the direction perpendicular to the surface of the base plate. As a result, it is not possible to use the epitaxial lateral overgrowth method to form only regions which have a low concentration of crystal defects. Therefore, in the technique of Japanese Laid-Open Patent Application Publication No. 2001-230410, a semiconductor layer having regions with a high concentration of crystal defects and regions with a low concentration of crystal defects is formed. The regions with the high concentration of crystal defects and the regions with the low concentration of crystal defects are distributed in the semiconductor layer. In the technique of Japanese Laid Open Patent Application Publication No. 2001-230410, a source region, a channel forming region and a drift region are formed within the region with the low concentration of crystal defects. The source region, the channel forming region and the drift region are stacked in the direction of thickness of the semiconductor layer. Furthermore, a trench is formed in the region with the high concentration of crystal defects, and a gate electrode is located within the trench. Moreover, a source electrode is formed at a top surface of the semiconductor device, and a drain electrode is formed at a bottom surface of the semiconductor device. This allows a semiconductor device with a vertical electrodes structure to be obtained in which the source electrode and the drain electrode are located separately at the top surface and the bottom surface respectively of the semiconductor device.
With the technique of Japanese Laid-Open Patent Application Publication No. 2001-230410, the source region, the channel forming region and the drift region are stacked in a vertical direction to form a vertical semiconductor structure in the region with the low concentration of crystal defects. Since the semiconductor structure can be formed in the region having the low concentration of crystal defects, the withstand voltage of the semiconductor device can be increased.
However, crystal defects are also present in the region with the low concentration of crystal defects that has been manufactured by the epitaxial lateral overgrowth method. Most of these crystal defects extend in the direction of thickness of the semiconductor layer. That is, with the semiconductor device of Japanese Laid-Open Patent Application Publication No. 2001-230410, the direction in which the source region, the channel forming region and the drift region are stacked is parallel to the direction in which the crystal defects extend. As a result, the direction of the electric field of the vertical semiconductor structure is parallel to the direction in which the crystal defects extend. When these two are parallel, the crystal defects can readily affect the characteristics of this vertical semiconductor structure.
Alternatively, the formation of a horizontal semiconductor structure could also be considered, in which the source region, the channel forming region and the drift region are distributed in a horizontal direction along the plane of the semiconductor layer. In this case, the direction of an electric field of the horizontal semiconductor structure is orthogonal to the direction in which the crystal defects extend. When these two are orthogonal, the crystal defects cannot readily affect the characteristics of the horizontal semiconductor structure. However, when the horizontal semiconductor structure is used, a semiconductor device is obtained in which both main electrodes are present on the top surface of the semiconductor device (in the present specification, this electrodes structure will be termed a horizontal electrodes structure).
Although a vertical electrodes structure can be found in the semiconductor device of Japanese Laid-Open Patent Application Publication No. 2001-230410, the direction of the electric field of the vertical semiconductor structure is parallel to the direction in which the crystal defects extend. As a result, the characteristics of the vertical semiconductor structure can readily be affected by the crystal defects. For example, in the drift region that has the purpose of increasing withstand voltage, the electric field that is formed when the semiconductor device has been turned off is parallel to the direction in which the crystal defects extend, and consequently the presence of the crystal defects reduces the withstand voltage of the semiconductor structure. Furthermore, crystal defects are also present in the channel forming region, these crystal defects extending from the source region towards the drift region. As a result, current leakage via the crystal defects occurs in the channel forming region that has the purpose of controlling the on/off of electric current.
If the horizontal semiconductor structure is adopted, the direction of the electric field of the horizontal semiconductor structure is orthogonal to the direction in which the crystal defects extend. As a result, it is possible to obtain a structure in which the presence of crystal defects cannot readily affect the characteristics of the horizontal semiconductor structure (for example, withstand voltage characteristics or current leakage characteristics). However, when the horizontal semiconductor structure is adopted, the semiconductor device has the horizontal electrodes structure, and the vertical electrodes structure cannot be realized.
The present invention aims to adopt a horizontal semiconductor structure, wherein the presence of crystal defects does not readily affect characteristics of the semiconductor structure, while realizing a semiconductor device with a vertical electrodes structure, in which it is simple to maintain insulation between the pair of main electrodes and in which it is easy to simplify the wiring layout connected to the pair of main electrodes.